JPS59126719A - Production of high tension hot rolled steel sheet having excellent processability - Google Patents

Abstract

PURPOSE:To obtain a high tension hot-rolled steel sheet having excellent processability by heating a billet contg. specific amt. of C, Mn, Si, P and S, subjecting the billet to continuous hot finish rolling under a specific condition thereby forming the composite structure of fine ferrite and bainite. CONSTITUTION:A billet is composed, by weight %, of 0.02-1.5 C, 0.3-1.5 Mn, <=1.5 Si, <=0.02 P, <=0.01 S, and the balance Fe with inevitable impurities. Such billet is heated and is subjected to continuous hot finish rolling at >=40% draft in each pass of the first >=4 passes and >=80% total draft in the entire finish rolling. The final rolling pass is finished at (Ar3+50 deg.C)-(Ar3-50 deg.C), and the steel sheet is cooled at a cooling rate of >=45 deg.C/sec upon ending of the rolling and is coiled at 350-550 deg.C.

Description

Detailed Description of the Invention The present invention uses C-Si-Mn steel or 0-Mn, does not require the addition of special elements (carbonitride formation such as Nb%Ti, V... By improving the tensile strength, it has a particularly high strength = ductility balance and excellent workability compared to conventional precipitation-strengthened high-strength hot-rolled steel sheets.

The present invention relates to a method for producing a high-tensile hot-rolled steel sheet and sometimes a high-tensile hot-rolled steel sheet that is excellent in processing with good elongation and frizzability.

In recent years, in the automobile industry, one of the measures to improve the fuel efficiency of automobiles is to reduce the weight of car bodies, and the demand for high-strength hot-rolled steel sheets with excellent workability has increased from the viewpoint of thinner steel sheets and safety. There is.

Conventionally, Nb,
A manufacturing method that improves strength through bath hardening and precipitation strengthening by carbonitride formation by adding special elements such as Ti, V...
(Dual Phase) There is a manufacturing method that improves the strength-ductility balance by using a steel plate. In the former, additive elements such as Nb1Ti1V... are expensive, causing an increase in cost, and future resource constraints are expected. Although the latter has a good strength-ductility balance, it is prone to cracking during automatic military fabrication and hollow hole forming by burr cutting. Due to the large softening phenomenon of the O-weld heat-affected zone, the wall thickness of that part decreases greatly when the wheel rim and mold are straightened.
There are two points: no improvement in fatigue properties is observed.

In view of the current situation, the present invention has been developed for general purpose 0-0-8i-
In particular, tensile strength of 50 kg/
To produce at low cost a high-tensile hot-rolled steel sheet with grade 111112 or higher, good elongation and weldability, and excellent workability, and especially a high-tensile hot-rolled steel sheet with good stretchability and weldability, and excellent workability. A new manufacturing method was developed for this purpose.

The present invention will be explained in detail below.

In the present invention, the basic components are C: 0.02 to 1.5
A steel piece is used in which the content is limited to Mn: 0.3 = 1.5%, st: 1.5% or less, P: 0.02% or less, S: 0.01% or less, and the balance is Fe and unavoidable elements. C
and Mn to ensure the necessary strength and to obtain a composite structure of ferrite and bainite (including some burrite, martini, and bainite) or bainite structure (in addition to the bainite phase, it also contains ferrite called ferrite) It is an essential element for C: less than 0.02%, Mn: 0.3
%, it is difficult to obtain a steel plate with a composite structure of ferrite and bainite or a steel plate with a bainite structure at normal hot cooling speed (10oc/s), and c:o,is%
As mentioned above, if In: 1.5 or more, the deterioration of ductility is large and the weldability is also impaired, so C: 0.02 to 0.15%,
Mn: 0.3: to 1.5%. Si is preferably 0
．． When added in an amount of 2% or more, the solid solution [C] within the ferrite grains decreases and promotes the concentration of C element in the untransformed austenite grains, making it easier to obtain a suitable composite structure of ferrite and bainite. Improves the strength-ductility balance of steel sheets.Si: 1.5% or more is economically disadvantageous because the C element concentration in untransformed austenite grains becomes saturated, and it impairs weldability. The content should be 0.5% or less, preferably 0.2 to 1.5%. Since P impairs weldability, the P content should be 0.2% or less. S forms MnS-based inclusions and reduces stretchability. Because it lowers M
S: 0.01% or less in order to reduce nS-based inclusions and improve stretchability. Oa has the function of controlling the shape of inclusions to make them into fine spheres, and improves elongation and frizzability, so it is preferably contained up to 0.01% Ca. Preferably, the heating temperature of the steel slab during hot rolling is 1200C or less.

This is done in order to make the austenite grains as small as possible during heating, and when the heating temperature is high, there is a delay of one hour at the end of the rolling speed or on the entry side of the finish rolling to ensure the final rolling bath temperature, which improves productivity. This is to reduce and prevent it. The rolling reduction rate for the first four passes or more of finish rolling is 40% each.
The reason why we set the rolling reduction ratio to be as high as possible is because the passing time of the steel plate between passes is considered to be sufficient time for the austenite grains to recrystallize after rolling. The objective is to increase the recrystallization nucleation rate and refine the austenite grains. After the 5th pass, since the time between passes is short, the effect of cumulative reduction (there is almost no recovery after each pass, and it can be evaluated as an accumulation of reduction) is considered, and the total reduction rate (finishing The rolling reduction ratio before and after rolling shall be 80 inches or more.

In the invention of 8g1, the temperature of the final rolling pass was set to (Ar3+
50C) to (Ara-50C) is achieved by applying a large pressure and, in this temperature range, refining the recrystallized austenite grains or preventing recovery of the deformation band introduced into the unrecrystallized austenite grains. This is to make the grains finer. Further, even if the transformation point is exceeded during rolling, recrystallization of ferrite has been observed to occur due to large reduction, which is effective for grain refinement. Furthermore, the two-phase separation of ferrite and austenite is promoted, and appropriate ferrite and bainite structures can be obtained. Ar3 here refers to the ferrite transformation temperature during cooling without rolling. Therefore, although it varies depending on the cooling rate and component system (
Ar3+50C) to (Ara-5oc) is approximately 850 to 750C.

FIG. 1 shows the influence of the temperature of the final rolling pass and the effect of the reduction rate in finish rolling on ferrite grains. When the temperature of the final rolling pass is 830 to 780C, the ferrite grains are smaller than when the temperature is 850C or higher. Moreover, even if the temperature after the final rolling is 830 to 780 C, it can be seen that the ferrite grains are further reduced by reducing the rolling reduction ratio of 40 to 50 inches in each of the first four baths of finish rolling. . That is, it can be inferred that the temperature of the final rolling pass and the rolling reduction rate of finish rolling have a large synergistic effect on the refinement of ferrite grains.

It can be seen that there is an appropriate range for these.

After rolling, the paper is cooled from the two-phase coexistence region to a temperature of 350 to 550C at a cooling rate of 45r1'/s or higher (including water injection cooling, air/water cooling, and 5-scene cooling) to produce a proper composite broken paper of ferrite and bainite. The goal is to make it easier to obtain. If the temperature of the final rolling pass is in the two-phase coexistence range, two-phase separation will proceed even if the product is immediately cooled after rolling, resulting in an appropriate composite structure of ferrite and bainite. However, when manufacturing thin and wide steel sheets with a composite structure of ferrite and bainite, the temperature range of the final rolling pass is (Ar3+50C) ~
(Ara -50C) For products that are difficult to manufacture because the rolling load exceeds the limit value of the rolling mill, the final pass temperature is set to (Ar3 + 50C) or higher (Second invention) After the final m rolling pass, the material is slowly cooled to the two-phase region temperature at a cooling rate of less than 45 C/S, and thereafter the cooling rate is 4 s C.
/S or higher to a temperature of 350 to 550C. This has insufficient grain refinement and is slightly inferior in strength-ductility balance.

The above is mainly a description of the method for manufacturing a steel sheet with a fine-grained ferrite and bainite structure according to the invention of claim 1 and a composite structure steel sheet having a ferrite and bainite structure according to the invention according to claim 2.

Next, to explain the production of a single-phase bainitic steel plate having a bainite area ratio of 70 inches or more, that is, the invention of claim 3, there is no need to particularly regulate the reduction rate in finish rolling, and the temperature of the final rolling pass is Ar3 or more. shall be. Preferably, the processed austenite grains complete recrystallization in a temperature range, and after the final rolling pass, cooling is started from a temperature of Ar3 or higher, and the cooling rate is 45 C/S or higher to 350 C/S.
A single-phase bainite structure is obtained by cooling to ~550C. In any of the above manufacturing methods, the cooling rate is 45 C/S or more, and the formation of ferrite/pearlite structure is organized by the cooling start temperature and cooling end temperature after the final rolling pass.

The characteristics of the hot-rolled steel sheet with a composite structure of ferrite and bainite produced by the method described above are that after two-phase separation, the strength is increased by the bainite structure, and the ductility is improved by making the ferrite into fine grains. As a result, it has properties that are far superior in strength-ductility balance than steel sheets with ferrite and pearlite structures obtained under normal manufacturing conditions and precipitation-strengthened high-strength hot-rolled steel sheets obtained by adding elements such as Ti1V and Nb. ing. In addition, single-phase bainite hot-rolled copper sheets with a bainite area ratio of 70q6 or more are inferior to ferrite-bainite composite structure hot-rolled steel sheets in terms of strength and ductility, but they are inferior to hot-rolled steel sheets with a composite structure of ferrite and bainite. It has excellent characteristics. Single-phase bainitic steel is softer than the bainite phase of woven steel than steel with a composite structure of ferrite and bainite, and homogeneous deformation occurs up to the high deformation range. This is thought to be because the bainite phase is extremely hard compared to the ferrite deformation and cannot follow the deformation of the ferrite, resulting in delamination at the interface due to destruction of the bainite phase or generation of voids. Figure 3.4.5 shows a composite structure of fine grained ferrite and bainite produced under the conditions of claim 1.2.30, a composite structure of ferrite and bainite with insufficient fine grains, and a single-phase bainite structure. The characteristic values of hot rolled copper sheets are compared and organized. Third
The figure shows the elongation of these hot-rolled steel sheets organized by final rolling pass temperature. It can be seen that the hot-rolled steel sheet with a composite structure of fine-grained ferrite and benite has the best elongation. FIG. 4 shows a comparison of the strength-ductility balance of these hot-rolled steel sheets with dual-phase hot-rolled steel sheets and precipitation-strengthened high-strength hot-rolled steel sheets.

A hot-rolled steel sheet with a composite structure of fine-grained ferrite and bainite is inferior to a dual-phase hot-rolled steel sheet in terms of strength-ductility balance, but is far superior to a precipitation-strengthened high-strength steel sheet. It is also seen that both the hot-rolled steel sheet with a composite structure of ferrite and baseite with insufficient fine grains and the hot-rolled steel sheet with a single-phase bainite structure have the same strength-ductility balance as the precipitation-strengthened high-strength steel sheet. FIG. 5 shows the hole expansion ratios of these hot-rolled steel sheets organized by Si. As the St content decreases, the hole expansion rate improves. Microstructure-wise, hot-rolled steel sheets with a single-phase bainite structure have a better hole expansion rate than hot-rolled steel sheets with a composite structure of ferrite and bainite, and their strength increases. Target 40-45
kg/v! It can be seen that the hole expansion rate is equivalent to that of .

Examples Examples according to the present invention are shown in Table 1. Inventive Examples 1 to 28 are obtained by hot rolling steel having predetermined components according to the first, second, and third aspects of the present invention. In Comparative Examples 29 to 40, steels having the same composition were rolled under hot rolling conditions different from the conditions of the present invention. In addition, as a comparative example, the strength-ductility balance of a precipitation-strengthened high-strength hot-rolled steel sheet and a dual-phase hot-rolled steel sheet by addition of 8b, which are known in the art, are shown in FIG.

Examples of the present invention will be described in detail below. Invention examples 1-1
5 is an invention example relating to the production of a steel plate with a fine-grained ferrite and bainite composite structure which is a steel having a predetermined composition and has excellent strength-ductility ratio. The rolling reduction rate up to the bath is 40 inches each, and the temperature of the final finishing pass is (Ar3+50C) ~ (Ara 50tr).
) (Actual performance: 845-755C), and thereafter from 2 phase area to 4
5 C/S-350-550C at a cooling rate of 80C/S
A coil that has been cooled by water injection (actual cooling is done from the ROT, front stage, or middle stage) to a temperature of .

Inventive Example 10.11, the rolling reduction rate for the first two passes of finish rolling is 50 inches, and the rolling reduction rate for the second and subsequent passes up to the fourth bus is 40 inches.
%, the temperature of the final finishing pass is (Ara+5 QC) ~ (A
RA 5 QC) (Actual performance: 800C), and thereafter from the two-phase region to 450C with a cooling rate of 45ClF3 and 60C/S.
A coil that has been cooled by water injection to a temperature of C and then wound. Invention example 12.
In No. 14, the composition was different from Invention Examples 1 to 11. The rolling reduction rate in the first four baths of finish rolling was 40%, the temperature of the final finishing bath was C and R9oc), and thereafter, the rolling reduction was 45 C/9 from the two-phase region. The coil was wound by water injection cooling to a temperature of 450 to 350 r at a cooling rate of S and 80 C/s. Invention example 13.15 is invention example 1
With the same ingredients as in 2.14, the rolling reduction rate up to the first two passes of finishing rolling is 50%, the rolling reduction rate from the second pass to the fourth pass is each 40%, and the temperature of the final finishing pass is (Ar3+50C: l~
(Ara-soc) completed (achievements 805C and 795C)
), after which the two-phase region was cooled by water injection at cooling rates of 45 C/S and 80 C/S to temperatures of 450 C and 350 C, and then rolled up. Further, Invention Examples 16 to 20 have the same composition as Invention Examples 1 to 11, and have a strength variation equivalent to that of a precipitation-strengthened high-strength hot-rolled steel sheet.Although they do not have sufficient fine grains, they have a composite structure of ferrite and bainite. Examples 16 to 19 of the invention relate to the production of hot-rolled steel sheets. Invention examples 16 to 19 have a rolling reduction rate of 40% in each of the first four nozzles of finish rolling, and a temperature of Ar3 or higher in the final finishing bath (actual results of 885 to 855C). ), after rolling, the coil was slowly cooled to a two-phase region temperature, and thereafter cooled by water injection to a temperature of 350 to 550C at a cooling rate of 45 to 60 t:/S, and then wound. Inventive example 20, the rolling reduction rate up to the first two passes of finish rolling is nominally 50%, and the rolling reduction rate from the second bus to the fourth pass is 4 each.
0chi, finish the final finishing pass at a temperature of Ar3 or above (Actual performance: 8
95C), after rolling, it was slowly cooled to a temperature in the two-phase region, and then 4 sC
Coil cooled by water injection to 4500 at a cooling rate of /S and wound. Further, invention examples 21 to 28 are invention examples relating to the production of hot rolled steel sheets having a single-phase pearlite structure with excellent stripe elongation/flange properties, of which invention examples 21 to 26 are invention examples 1 to 1.
The same ingredients as 1, finished at the final finishing bath temperature Ara or higher (actual results 855-910G), and after rolling Ar3 or higher to 4
A coil cooled by water injection to a temperature of 350 to 550C at a cooling rate of 5 to 65C/S and then wound. Invention examples 27 and 28 have the same components as invention examples 12 to 15, and the temperature of the final finishing bath is set to Ar.
Finished with 3 or more (actual results 870C and 880C), Ar after rolling
A coil that is wound by water injection cooling from 3 or more to a temperature of 450C at a cooling rate of 45C/S. and the tensile strength is 50~
60kg/w! Therefore, materials with an elongation of 28 inches or more ξ (examples of the present invention, first invention - second invention) and a hole expansion ratio of 1.5 or more were obtained (・ru.Compared to these, Comparative Examples 29 to 38 are Invention Examples 1 to 11 and the first component, among which Comparative Example 29.30 had a cooling rate of 15 C/S and 35 ClF after rolling.
3 and slow (・), an appropriate composite structure of ferrite and bainite cannot be obtained, and a slight decrease in strength-ductility parasis is observed.

In Comparative Example 31.32, the rolling reduction in the first four passes of finish rolling was as small as 30%, so proper fine grain ferrite could not be obtained, and the strength-ductility J'; 5:7 SiJ''-decreased. In Comparative Examples 33 and 34, the temperature after the water injection cooling was completed was 560℃.
Since the C4600C is high (・), it becomes a ferrite and pearlite structure, the crystal grains are large, and the strength is reduced.In Comparative Examples 35 to 38, the temperature after cooling by water injection was low, so a composite structure of tesitite was formed by combining ferrite and macerate. Compared to Si steel, the C purification of ferrite is not captured, so the strength is uneven and the elongation is significantly deteriorated.
Ru.

Furthermore, although the dual-phase hot rolled steel sheet of the comparative example shown in FIG. It's also expensive. Further, the precipitation-strengthened high-strength hot-rolled steel sheet of the comparative example shown in FIG. 4 is inferior to the hot-rolled steel sheet of the example of the present invention in terms of strength-ductility variation, and is also more expensive due to the addition of a special element (Nb).

From the above results, according to the present invention, the tensile strength is 50
As described above, we have created a high-strength hot-rolled steel sheet that has superior strength-ductility balance compared to conventional precipitation-strengthened high-strength hot-rolled copper sheets and has a hole expansion ratio of 1.5 or more and excellent workability and weldability without the addition of special elements. (It can be manufactured at low cost using current equipment, and the industrial profits are large.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the temperature of the final rolling bath, the reduction ratio of F1 to F4 in finish rolling, and the grain size, and Figure 2 shows the relationship between the cooling start temperature, cooling end temperature, and microstructure of the final rolling bath. Figure 3 is a diagram showing the relationship between final rolling bath temperature and elongation, Figure 4 is a diagram showing a comparison of strength-ductility variation, and Figure 5 is the relationship between S content, structure, and hole expansion ratio. FIG. Figure 11 1shi final pressure 舊F) Gusu no Toton! ! Lπ) No. 2p No. 3 A-WtbAli ("Cr No. 4 No. 31) No. 1゜Name (Name) (665) New 1 Steel Corporation Co., Ltd. 4, Agent address: 330 A, Shigesu Building, 2-6-2 Marunouchi, Chiyog1-ku, Tokyo 100- Amendment We will amend the following matters in the specification. The scope of claims will be corrected as shown in the attached sheet. - What is 0.02-0.15 in line 14 of page 1:0.02-1.5ch? This is corrected as "C".Claim 1 00.02-0.15%, In 0.3-1
．． 5%, SL<1.5ch, P<:0.02%, Skuo
, oll, with the balance consisting of Fe and unavoidable elements, and the rolling reduction ratio in each of the first four or more passes of continuous hot finishing rolling is 40% or more, and the total rolling reduction of the entire finishing rolling is ratio is 80% or more, and the temperature of the final rolling bath is (
Ara+500)~(Ar35(F:)), cooled at a cooling rate of 45℃/S or more after finishing, 350℃~55
A method for producing a high tensile strength hot rolled steel sheet having a composite structure of fine ferrite and bainite, good strength-ductility balance, and excellent workability, characterized by winding at 0°C. 2. Heat the steel billet, make the rolling reduction in each of the first 4 or more buses of continuous hot finishing rolling 40% or more, make the total rolling reduction in all finishing rolling 80% or more, and set the temperature of the final rolling bath to 40% or more. (Arg + s o ℃) or more, and after finishing, A
Slowly cool at a cooling rate of less than 45VS from r3 to Ar1 temperature, then cool at a cooling rate of 45℃ δ or higher, and cool from 350℃ to 550℃.
A method for producing a high-tensile hot-rolled steel sheet with excellent workability according to claim 1, characterized in that the sheet is wound at C. 3. Heat the steel billet and finish at Ar3 or higher, which is the temperature of the final rolling bath of continuous hot finish rolling. After finishing, start cooling from Ar3 or higher and cool at a cooling rate of 45°C/S or higher to 350°C.
Bainite area ratio 70 by winding at ℃~550℃
% or more of single-phase bainitic composition with elongation flora,
A method for manufacturing a high-tensile hot-rolled steel sheet with excellent workability as claimed in claim 1, characterized in that the steel sheet has good 7-dimensional strength. Procedural amendment book Showa! ? May 3, 2017 - Relationship with the Patent Office Director General Futa Yuko Hidono Rachi Case Applicant's address (residence) Higashi h' + 'rl' Ote 11, Chiyo 11-ku
No. 12-6-3 Name (665) Nippon Steel Corporation 4, Agent Address No. 2-6-2 Marunouchi, Moyota-ku, Tokyo Shigesu Building 330 Amended Japanese application details The following matters in the buttocks and drawings will be corrected. 0 Note 1. The scope of claims will be corrected as shown in the attached sheet. 〇ゝ°2. From the bottom line of page 4 to the 7th line of page 5, it says “or bainite structure...
...1.5% or more'' means ``Also, it is essential to obtain a bainite-ferrite composite structure mainly composed of bainite (a composite structure containing a bainite structure and a reed + 1 ra-ferrite structure). is an element, C:
: less than 0.02%, Mn: less than 0.3%, normal H
At ot cooling speed, it is difficult to obtain a steel plate with a composite structure of ferrite and bainite, and C: more than 0.15%, Mn: 1
．． 5% or more.” 3. On the 7th line from the bottom of page 5, it says "rsi: 1.5 or more".
"It's more than that," he corrected. 4. On page 8, lines 10-11, on page 9, line 5, on the second line from the bottom, and on the second line of 10th line, it says "Cooling rate 45℃/S or more", respectively. ℃/S or more and around 100℃/S.'' 5. In lines 12-13 of page 9 and line 4 from the bottom of page 1O, the words "single-phase bainite" are corrected to "bainite-ferrite composite structure mainly composed of bainite." 6.Page 9, bottom line - page 10, lines 1 and 5, page 11, line 6 from Teishita, page 12, lines 9 to IO, line 5 from the bottom, and page 18, line 8 The words "single-phase bainite structure" have been corrected to "bainite-ferrite composite structure mainly composed of bainite." 7. In the 3rd and 4th lines of page 1O, the phrase "bainite structure" is corrected to "the above structure." 8. On page 11, lines 1 to 4, the phrase "single-phase bainite...bainitic steel" is corrected to "the reason for this is a bainite-ferrite composite structure steel consisting mainly of bainite."09. Page 15, Table 1-2 [In column 21j, "bainite structure" is corrected to "bainite-ferrite composite structure mainly composed of bainite." 10. In the drawings, "Figure 2,""Figure3,""Figure4," and "Figure 5" will be corrected to the drawings submitted at the same time today. Claims t C0.02~o, iss, Mno, 3~1.5%
, Si<1.5%, p<0.02%, S<10.01%
A steel billet containing Fe and unavoidable elements is heated, and the rolling reduction in each of the first four or more passes of continuous hot finish rolling is 40q6 or more, and the total rolling reduction in the entire finishing rolling is 80 The temperature of the final rolling pass was set to (Ar3
+50℃)~(Ar3-50℃), after finishing 45℃
It is characterized by being cooled at a cooling rate of ℃/S or more and coiled at 350℃ to 550℃.It has a composite structure of fine feite and bainite, has a strong and ductile burr system, and has excellent workability. A method for producing tension hot-rolled steel sheets. 2. Heat the steel billet, make the rolling reduction in each of the first 4 passes or more of continuous hot finishing rolling 40 inches or more, make the total rolling reduction in all finish rolling 80 inches or more, and set the temperature of the final rolling pass to 40 inches or more. (Ar3 + 50℃) or higher, and after finishing, Ar
3 ~ Slowly cooled at a cooling rate of less than 45°C/S until Ar processing temperature, then cooled at a cooling rate of 45°C/S or more, 350°C to 55°C
A method for producing a high-strength hot-rolled steel sheet with excellent workability as claimed in claim 1, wherein the method comprises winding at 0°C. 3 Heat the steel billet, finish the final rolling pass of continuous hot finish rolling at a temperature of Ar 3 or higher, and after finishing, start cooling from Ar 3 or higher and cool at a cooling rate of 45°C/S or higher.
Claim 1, characterized in that by winding at 50° C. to 550° C., the steel sheet is combined with a steel plate having a light composite structure having a pearlite area ratio of 70% or more and having good elongation frizzability. A method for manufacturing high-strength hot-rolled steel sheets with excellent workability. Fig. 2 Fig. 3 Fig. 11 Sakai Rolling L (J supervision IL fog) Fig. 4 Tensile strength -! (K8μ7g) Figure 5

Claims (1)

[Claims] I CO, 02-1.5%, In 0.3-1.5%
, St <1.5chi, Pkuo, o2chi, S<:0.01
A copper piece is heated, and the rolling reduction rate in each pass of the first four or more passes of continuous hot finishing rolling is 40% or more, and the total rolling reduction rate of all finishing rolling is heated. is 80 inches or more, and the temperature of the final rolling pass is (Ar3
Finish with +50 C) ~ (Ara-50C), and after finishing 4
Cool at a cooling rate of 5 C/S or higher, 350C to 550C
A method for producing a high-tensile hot-rolled steel sheet having a composite structure of fine ferrite and bainite, good strength-ductility balance, and excellent workability, characterized by winding at C. 2. Heat the steel billet, make the rolling reduction in each of the first 4 or more passes of continuous hot finishing rolling 40 inches or more, make the total rolling reduction in all finish rolling 80 inches or more, and set the temperature of the final rolling pass to 40 inches or more. Finish with (Ar3+50C) or more, and after finishing, Ar3～
Slow cooling at a cooling rate of less than 45 C/S until Ar1 temperature, then cooling at a cooling rate of 45 C/S or more, 350 C to 55
A method for producing a high-tensile hot-rolled copper plate having excellent workability as claimed in claim 1, wherein the copper plate is wound at 0C. 3. Heating the steel billet, finishing the final rolling pass of continuous hot finish rolling at a temperature of Ar3 or higher, and after finishing, cooling started from Ar3 or higher and cooling at a cooling rate of 45 C/s or higher.35
Bainite area ratio 7 by winding at 0C to 550C
A method for producing a high-strength hot-rolled steel sheet with excellent workability as set forth in item 1 of the claims, characterized in that the steel sheet has a single-phase bainite structure of 0% or more and has good elongation fracture properties.